Nitride semiconductors which contain nitrogen (N) as a group V element are a promising material for short-wavelength light-emitting elements because of their band gap sizes. Of the nitride semiconductors, gallium nitride-based compound semiconductors, in particular, are being actively studied, and have been put into practical use in the form of blue light-emitting diodes (LEDs), green LEDs, and semiconductor lasers that use a GaN-based semiconductor as a material (see, for example, Patent Documents No. 1 and 2).
Hereinafter, gallium nitride-based compound semiconductors are referred to as GaN-based semiconductors. GaN-based semiconductors include compound semiconductors that substitute at least one of aluminum (Al) and indium (In) for some or all of Ga atoms, and GaN-based semiconductors are expressed by a composition formula of AlxGayInzN (0≦x, y, z≦1, x+y+z=1).
Substituting Al or In for Ga for Ga makes the band gap larger or smaller than that of GaN. In this manner, not only GaN-based semiconductor elements that emit short-wavelength light such as blue light or green light but also GaN-based semiconductor elements that emit orange light or red light are obtained. A light-emitting element that emits light of any wavelength selected from the entire visible light range is therefore theoretically possible with the use of a GaN-based semiconductor, and the application of Gan-based semiconductor light-emitting elements to image display devices and illuminating devices is expected.
GaN-based semiconductors have a wurtzite crystal structure. FIG. 1 illustrates planes of the wurtzite crystal structure expressed in four-digit indices (Hexagonal Miller-Bravais indices). Four-digit indices use basis vectors denoted by a1, a2, a3, and c to express crystal planes and orientations. The basis vector c runs in a [0001] direction, which is called “c axis”. A plane perpendicular to the c axis is called as “c plane” or “(0001) plane”. The “c axis” and the “c plane” are sometimes written as “C axis” and “C plane”, respectively. FIG. 2(a) illustrates the crystal structure of a GaN-based semiconductor in the form of a ball-stick model. FIG. 2(b) illustrates the positions of Ga and N of the GaN-based semiconductor crystal on a plane perpendicular to the c axis.
Conventionally, when manufacturing a semiconductor element from a GaN-based semiconductor, a c-plane substrate, i.e., a substrate having a (0001) plane as a surface is used as a substrate on which a GaN-based semiconductor crystal is to be grown. As is understood from FIGS. 2(a) and 2(b), a layer in which Ga atoms alone are arranged and a layer in which N atoms alone are arranged are formed in the c-axis direction in this case. Because of this arrangement of Ga atoms and N atoms, spontaneous electrical polarization occurs in the GaN-based semiconductor. The “c plane” is therefore also called as “polarity plane”.
Consequently, a piezo-electric field is generated along the c-axis direction in a quantum well of InGaN in an active layer of the GaN-based light-emitting element, and shifts the distribution of electrons and holes within the active layer, thereby lowering the internal quantum efficiency of the active layer through quantum-confined Stark effect of carriers. The result is an increase in threshold current in the case of a semiconductor laser, and an increase in power consumption and a drop in luminous efficacy in the case of an LED. Further, a rise in injected-carrier density is followed by the screening of the piezo-electric field and a change in light emission wavelength.
If the In composition of the InGaN active layer is increased in order to make the semiconductor element emit green light, or light in the long-wavelength range such as orange light and red light, the intensity of the piezo-electric field increases even more along with the In composition and the internal quantum efficiency drops rapidly. It is therefore a general opinion that an LED using an InGaN active layer that has a c plane can only emit light whose wavelength is up to 550 nm or so.
As a solution to this problem, the use of a substrate having as its surface an m plane which is a non-polarity plane (an m-plane GaN-based substrate) has been considered in the manufacture of a light-emitting element. As illustrated in FIG. 1, m planes in the wurtzite crystal structure are parallel to the c axis and are six equivalent planes orthogonal to the c planes. For instance, a (10-10) plane hatched in FIG. 1 which is perpendicular to a [10-10] direction is an m plane. Other m planes equivalent to the (10-10) plane include a (-1010) plane, a (1-100) plane, a (-1100) plane, a (01-10) plane, and a (0-110) plane. The sign “-” to the left of a number inside the parentheses that indicates a Miller index means a “bar”.
FIG. 2(c) illustrates the positions of Ga and N of the GaN-based semiconductor crystal on a plane perpendicular to an m plane. Ga atoms and nitrogen atoms on an m plane exist on the same atomic plane as illustrated in FIG. 2(c) and, therefore, electrical polarization does not occur in a direction perpendicular to the m plane. Accordingly, using a semiconductor multilayer structure that is formed on an m plane in the manufacture of a light-emitting element prevents the generation of a piezo-electric field in the active layer, thereby solving the problem described above.
The solution also allows the In composition of the active layer to increase greatly, and thus makes it possible to produce an LED or a laser diode that emits light having a longer wavelength than blue light, such as green light, orange light, or red light, from the same material base that is used for a blue-light LED or laser diode.
Herein, an epitaxial growth in a direction perpendicular to an X plane (X=c, m) of a hexagonal crystal wurtzite structure is expressed as “X-plane growth”. In X-plane growth, the X plane is referred to as “growth plane” and a semiconductor layer formed through X-plane growth is referred to as “X-plane semiconductor layer”.
As disclosed in Non-patent Document No. 1 and others, an LED that uses an InGaN active layer formed on an m plane has optical polarization characteristics originated from the structure of its valence band. Specifically, the LED emits light that is polarized in a direction parallel to an a axis.
The LED is therefore expected to be used as a light-emitting element that can emit light having optical polarization characteristics. For instance, liquid crystal display devices which utilize the function of controlling the optical polarization direction of a liquid crystal need to use polarized light as a light source. In conventional liquid crystal display devices, there is no appropriate light source for emitting light that has optical polarization characteristics, and an LED, a cold cathode fluorescent lamp (CCFL), or a similar light source is thus arranged such that emitted light passes through a polarizing plate to obtain light that has optical polarization characteristics. However, this structure has a problem in that the polarizing plate blocks most of the light emitted from the light source and consequently lowers the light utilization efficiency. The LED that uses an InGaN active layer formed on an m plane, when employed as the light source of a liquid crystal display device, improves the light utilization efficiency, greatly reduces the power consumption of the liquid crystal display device, and eliminates the need for a polarizing plate, which lowers manufacturing cost.